Combustion of fossil fuels is reported to be a major cause of the increased concentration of carbon dioxide (CO2) in the atmosphere. Although research is ongoing to improve energy efficiency and to substitute low-carbon fuels to combat this problem, these methods will likely be insufficient to limit the growth of atmospheric CO2 concentrations to an acceptable level. As a result, there is tremendous interest in the development of methods for preventing CO2 release into the atmosphere, i.e., carbon capture and storage (CCS) technology.
A number of technologies are available for removing CO2 from a gaseous stream, including wet chemical absorption with a solvent (e.g., using amines such as monoethanolamine or diethanolamine), membrane separation, cryogenic fractionation, and adsorption using molecular sieves. Another method for the removal of CO2 from a gas stream involves dry scrubbing, meaning treatment of the process gas with a dry, regenerable sorbent that removes CO2 by chemical absorption/adsorption.
Existing technologies for CO2 capture from gaseous streams suffer from a number of drawbacks. The Department of Energy has reported that existing CO2 capture technologies are not cost-effective when considered in the context of large power plants. The net electricity produced from existing plants would be significantly reduced upon implementation of many of these CO2 capture technologies, since a high percentage of the power generated by the plant would have to be used to capture and compress the CO2. Additionally, the process conditions under which the CO2 must be removed in many applications render the existing technologies unusable. For example, exhaust gas streams including automotive exhaust, cement kiln flue gas, steel mill flue gas, diesel generator exhaust, and many other industrial and process gas streams are simply too hot (up to 600° C.) for conventional post-combustion CO2 capture technologies. Still further, the CO2 partial pressure of these gas streams is too low, typically less than 14.7 psia CO2, for natural gas sweetening or syngas CO2 capture technologies to be effective. The combination of high temperatures and low CO2 partial pressures makes the development of a material capable of effectively removing CO2 from these gas streams a significant challenge.
U.S. Pat. Nos. 5,480,625 and 5,681,503 are directed to sorbents for removing carbon dioxide from habitable enclosed spaces, the sorbents including a metal oxide (e.g., silver oxide) as the active agent and an alkali metal carbonate. However, the only exemplified sorbent regeneration temperature range given is 160-220° C., too low to be useful for most exhaust gas applications.
U.S. Pat. No. 6,280,503 describes a solid sorbent comprising magnesium oxide, preferably promoted with an alkali metal carbonate or alkali metal bicarbonate, for removal of CO2 from gas streams at temperatures in the range of 300 to 500° C.
U.S. Pat. No. 6,387,337 describes a CO2 capture system that utilizes a sorbent in the form of an alkali metal compound or an alkaline-earth metal compound, and that purportedly operates over a temperature range of 200 to 2000° F.
U.S. Pat. No. 6,387,845 is directed to a CO2-absorbing sorbent comprising lithium silicate optionally promoted by addition of an alkali metal carbonate, and which is capable of operation at temperatures exceeding about 500° C.
U.S. Pat. No. 7,314,847 is directed to a regenerable sorbent for CO2 capture that includes a binder in combination with one or more active components selected from alkali metal oxide, alkali metal hydroxide, alkaline earth metal oxide, alkaline earth metal hydroxide, alkali titanate, alkali zirconate, and alkali silicate. The sorbents are described as capable of operation over a temperature range of 25 to 600° C.
U.S. Pat. No. 8,110,523 describes a sorbent for CO2 capture that comprises an alkali metal carbonate or bicarbonate combined with a high surface area support and a binder. The patent suggests that the sorbent can operate over a temperature range of 40-200° C.
There is a continuing need in the art for the development of a sorbent material that is capable of effectively removing CO2 from gaseous streams, particularly exhaust gas streams characterized by relatively high temperatures and relatively low CO2 partial pressures.